Final answer:
The differences in the properties of the cuticle on the upper and lower surfaces of leaves could explain the differences in the rates of uptake of NAA. The thickness and impermeability of the cuticle, as well as the presence of trichomes, can affect the rate of transpiration and NAA uptake.
Step-by-step explanation:
The cuticle on the upper and lower surfaces of leaves have differences in their properties. The cuticle is a waxy layer that covers the leaves of all plant species and reduces the rate of water loss from the leaf surface. The differences in the cuticle on the upper and lower surfaces could explain the differences in the rates of uptake of NAA.
For example, if the cuticle on the upper surface is thicker, it might be more impermeable to NAA compared to the cuticle on the lower surface. This could result in a slower rate of NAA uptake by the upper surface compared to the lower surface.
Additionally, the presence of trichomes on the leaf surface could also affect the rate of NAA uptake. Trichomes can reduce the rate of transpiration by blocking air flow across the leaf surface. If the upper surface has more trichomes compared to the lower surface, it might experience a lower rate of transpiration and therefore a slower rate of NAA uptake.